Cryptography and Network Security Assignment - Detailed Analysis

This report analyzes a cryptography and network security assignment, focusing on the RSA encryption algorithm. The assignment involves a scenario where Eve attempts to intercept messages between Alice and Bob regarding a potential business deal. The analysis covers four messages: the first indicating a "yes" or "no" decision, the second representing a price, and the third and fourth involving text encryption using different methods (character-wise and word-wise). The report examines potential attacks Eve could use to retrieve the original messages, providing estimates of the time required for each attack. It then proposes changes to the encryption scheme to prevent such attacks, enhancing the security of the communication. The report provides insights into the vulnerabilities of the encryption scheme and suggests methods to make it more secure.

16COC140 Cryptography and Network Security
Coursework
Alice represents a company AAA that is considering buying Bob’s company,
BBB. Eve knows about the proposed transaction and she will be able to gain
financial advantage if she can find out as quickly as possible the details of the
deal. Eve can intercept the messages between Alice to Bob, but has no access to
their computers. She knows that they will communicate using the RSA public-key
encryption algorithm in its basic form, as described in the lectures. Eve has Bob’s
public key (e,n) where e=7 and n is a 256-bit integer. She does not have Bob’s
private key. Eve also knows that the messages will be constructed as follows:
1. An initial e-mail is sent by Alice to Bob to let him know of AAA’s
decision about trying or not to buy BBB. For a “no” decision, the message
is 0. For “yes”, the message is 1. The number 0 or 1, respectively, is then
encrypted using the RSA algorithm.
2. The next day, Alice sends Bob an e-mail to inform him of the proposed
price offered. The message consists of an integer number representing the
price in pounds. This number is encrypted using RSA. Eve estimates the
price to be between 50 million and 60 million pounds.
3. A third message is sent on the third day from Alice, consisting of a longer
text (say 5,000 characters) discussing some initial details of the deal. For
this e-mail, she uses the following method of encryption: each individual
character of the text is viewed as the integer corresponding to the ASCII
code of that character. The number thus obtained for each character is then
encrypted using the RSA algorithm, and the resulting list of numbers is
sent.
4. The following day Alice again writes a long e-mail (say 5,000 characters)
discussing further details of the deal. For this e-mail, she uses the
following method of encryption: each single word of the text is viewed as
an integer number in binary, obtained by concatenating the 8-bit binary
values of the ASCII code of each letter (including leading zeros). The
number thus obtained for each word is then encrypted using the RSA
algorithm and the resulting list of numbers is sent.
i. Discuss whether Eve can find efficient methods of retrieving each of the four
original messages.
[10 marks]
ii. For each attack you identified above, give a rough estimate of the time Eve
would need to succeed.
[3 marks]
iii. For each of the attacks you identified above, propose changes to the
encryption scheme to prevent such an attack.
[7 marks]
PTO